1. Field of the Invention
The invention relates to a four-wheel drive vehicle that enables switching between a four-wheel drive state where a driving force of a driving source is transmitted to main driving wheels and auxiliary driving wheels and a two-wheel drive state where the driving force of the driving source is transmitted only to the main driving wheels, and relates to a method for controlling the four-wheel drive vehicle.
2. Description of Related Art
Known four-wheel drive vehicles that enable switching between a four-wheel drive state and a two-wheel drive state are described in Japanese Patent Application Publication No. 2010-100280 (JP 2010-100280 A) and Japanese Patent Application Publication No. 2011-149535 (JP 2011-149535 A).
JP 2010-100280 A describes, as a sixth embodiment, a four-wheel drive vehicle including a driving unit that generates a driving force, a propeller shaft (torque transmission section) that receives, via a torque distribution device, the driving force generated by the driving unit, a dog clutch that enables cutting off of torque transmission from the driving unit to the propeller shaft, a pair of multidisc clutches including a plurality of clutch discs that enable adjustment of a torque transmitted from the propeller shaft to right and left rear wheels, and a control unit that controls the dog clutch and the multidisc clutches.
In the two-wheel drive state of this four-wheel drive vehicle, the control unit performs control to cut off the driving force transmission through the dog clutch and also cut off the driving force transmission through the multidisc clutches. As a result, the driving force transmission is cut off both the front wheel side and the rear wheel side of the propeller shaft, and rotation of the propeller shaft is stopped. Thus, when the rotation of the propeller shaft is stopped during traveling in the two-wheel drive state, traveling resistance is reduced to enhance the fuel efficiency of the four-wheel drive vehicle.
The four-wheel drive vehicle described in JP 2011-149535 A includes a pair of hydraulic clutches that transmits the rotational torque of the propeller shaft to the right and left rear wheels. An actuation circuit that actuates the hydraulic clutches includes an oil pump that supplies hydraulic oil and a pressure regulating valve that adjusts hydraulic pressure acting on pressing components that press the hydraulic clutches. Each of the hydraulic clutches has multiple clutch plates (frictional engagement components) that can be frictionally engaged with one another. The clutch plates are pressed by the pressing component and frictionally engaged with one another.
When the four-wheel drive vehicle described in JP 2010-100280 A is switched from the two-wheel drive state to the four-wheel drive state, first, a frictional force is generated between the clutch discs of the multidisc clutch to transmit the turning force of the right and left rear wheels to the propeller shaft. The propeller shaft rotates to achieve rotational synchronization at the dog clutch, and then the dog clutch is coupled to the propeller shaft. Thus, a state is set up in which the driving force generated by the driving unit can be transmitted to the right and left rear wheels, that is, the four-wheel drive state is set up.
On the other hand, during traveling in the two-wheel drive state where the rotation of the propeller shaft is stopped, the clutch discs in the multidisc clutch rotate relative to one another. When the relative rotation causes a drag torque, the effect of enhancing the fuel efficiency may not be sufficiently exerted. For a reduced drag torque, increasing the gap between the clutch discs is effective. However, in this case, coupling and uncoupling responsiveness of the clutches may be degraded.
The trade-off between the reduction in the drag torque of the clutches and the responsiveness of the clutches may similarly result from the hydraulic pressing of the plurality of clutch plates as in the technique described in JP 2011-149535 A. That is, there has also been a desire to achieve both a reduction in drag torque and enhancement of the responsiveness to switching from the two-wheel drive state to the four-wheel drive state when, for example, the multidisc clutches in the four-wheel drive vehicle described in JP 2010-100280 A are hydraulically controlled.